The miniaturization of electronic devices has been accompanied by advances in compactness and performance of the electronic components used, and the assembly of electronic devices often involves automatically mounting the electronic components onto a printed circuit board. Generally, electronic components for surface mounting are stored and transported in the form of electronic component packages formed by encasing them in carrier tape having consecutively embossed pockets for housing the electronic components, overlaying a cover film on the tape surface of the carrier tape to form a lid material, and continuously heat sealing with a seal bar. The cover film may be of a type having a sealant layer formed of a thermoplastic resin laminated over a base material comprising a biaxially stretched polyester film.
When mounting electronic components during the production of electronic devices, the cover film is peeled from the carrier tape using an automatic peeling device, the electronic components housed in the carrier tape are extracted by a pickup device, and mounted on the surface of an electronic circuit board. For this reason, it is particularly important for the peel strength of the cover film with respect to the carrier tape to be made stable within an appropriate range. If the peel strength is too high, the cover film can tear during peeling, and if too weak, the cover film can come away from the carrier tape during storage or transport, and the electronic components contained therein may be lost. In particular, the rapid advances in mounting speed have led to very high cover film peeling rates of 0.1 seconds per tact or less, which places great stress on the cover film during peeling. As a result, there is a problem called “film breakage” in which the cover film is torn.
The peel strength required to peel the cover film from the carrier tape is defined under the JIS C0806-3 standard as 0.1 to 1.0 N for 8 mm carrier tape and 0.1 to 1.3 N for 12 mm to 56 mm carrier tape, when the peeling rate is 300 mm per minute. However, during the actual process of mounting electronic components, the peeling rate is faster than 300 mm per minute, and particularly when housing large connector components, they are often sealed with a peel strength close to the upper limit, as a result of which film breakage is liable to occur when peeling the cover film.
As a countermeasure against film breakage, a method of providing an intermediate layer excelling in impact resistance and tear propagation resistance such as polypropylene, nylon or polyurethane between a substrate of biaxially stretched polyester film or the like and a sealant layer has been proposed (see Patent Documents 1-3). On the other hand, a method of using a metallocene linear low-density polyethylene (m-LLDP) of a certain specific gravity as the intermediate layer and further giving the adhesive layer between the intermediate layer and the substrate layer a low Young's modulus to prevent the propagation of stresses to the substrate layer has also been proposed (see Patent Document 4). Additionally, a method of using a mixture of polyethylene or polypropylene with a styrene-butadiene-styrene block copolymer as the sealant resin composition for the purpose of obtaining a cover film with stable sealing properties having low sealing temperature dependence and change over time of the peel strength has been proposed (see Patent Document 5). However, even with these methods, it is difficult to avoid film breakage during high-speed peeling of 100 m per minute.
Furthermore, a cover film with improved interlayer adhesive force by coextrusion of layers formed of styrene hydrocarbon resins has been proposed (see Patent Document 6). However, even with this method, the stability of the peel strength after heat sealing is inadequate.
Additionally, during high-speed peeling, there is extensive generation of static electricity due to peeling, requiring prevention of electrostatic damage during peeling.
Techniques for reducing the generation of static electricity during peeling include a technique of kneading conductive powders such as conductive carbon particles and metal oxides or metal microparticles into the sealant layer of the cover film (see, e.g., Patent Document 1). However, since metal oxides are relatively expensive, they can easily lead to increased costs, and since it is not easy to uniformly disperse metal oxides into the sealant layer, dispersion problems can cause disparities in peel strength.
Additionally, while the dispersion of surfactants into the sealant layer (see Patent Document 7) has been proposed, when an electronic component package formed of a carrier tape heat sealed by a cover film is stored for days in a high-temperature, high-humidity environment, the surfactant can migrate to the surface of the sealant layer, reducing the peel strength and causing the cover film to peel.
Furthermore, a method of not mixing conductive microparticles and the like into the sealant layer but providing a charge transfer layer between the substrate layer and the sealant layer to prevent static electricity from accumulating on the sealant surface layer has been proposed (see Patent Document 8). Such methods of heat sealing by means of a sealant layer not containing foreign articles such as conductive microparticles are intended to stabilize the peel strength, and in that sense, they are highly effective. However, the miniaturization of electronic components has led to a demand for even higher levels of suppression of static electricity generated on the surface of the sealant layer, and the demanded performance may not be achievable.
In addition to the above properties, the cover film must have high transparency to enable the contained electronic components to be easily identified. For example, inspection of electronic components such as IC's often involves identifying defects such as deformation of IC pins by image analysis using a CCD camera positioned above the cover film, in which case a highly transparent cover film is necessary. In order to efficiently perform such identification, the cover film must have a haze value of 50% or less and a total transmission of at least 75%.    Patent Document 1: Japanese Patent No. 3241220    Patent Document 2: JP H10-250020A    Patent Document 3: JP 2000-327024A    Patent Document 4: JP 2006-327624A    Patent Document 5: JP H8-324676A    Patent Document 6: JP 2007-90725A    Patent Document 7: JP 2004-51106A    Patent Document 8: JP 2005-178073A